Anyone able to tell me more about this?

This is one of the guns I inherited from my father, and it's the one that I know very little about. So far, I've been told that it was a black powder converted to a percussion, and it looks to me that the proof marks indicate Belgian made some time in the 1800's. The firing pin comes all the way out, but it fits so well it seems to be the original. The loading mechanism is most interesting - it flips open, but it goes sideways. Snaps really well; I have not tried to fire it nor do I plan on it, but it certainly seems to have the capability. (If it does fire, I bet it kicks like a mule!) The numbers 298 are stamped on the side of the butt and they appear to be mechanically done, although they're very faded.

Plenty of proof marks - hard to photograph though - the ECG in the circle on the barrel, there's a "P" inside a circle with a "V" inside another circle in several places, an "M" with a star above it inside a shield (that one's right below the hammer), an "M" inside a diamond, a "D" with a star above it, and a column on top of the chamber, and all the screws have either the P/V symbol or else what looks like a lower-case, cursive-g squiggly. A really good cleaning would help, but I'm quite aware that that's not a good thing to do, so I've just used some gun oil on it and it's been very lightly cleaned the same way I clean my other guns. I haven't had any luck uploading photos here, but here's what I've got on photobucket:

French 1853/1857 muskets, converted to breechloading. Belgian barrels
were used in many of the shotgun conversions.

Interesting period in history. The advent of the self contained cartridge,
and everyone stuck with literally PILES of obsolete muzzle loader muskets.
The British came up with the Snider conversion for the Enfield, the US
had the Allin conversion for the Springfield--and the French had the Tabatier.

Just FWIW, "tabatiere" is French for "snuff box", so called because the flip open breech resembled the lids of the snuff boxes popular at the time (1860's). The French converted many muskets (I believe that was a Mle. 1857) to that system before abandoning it for the Chassepot. When the muskets were sold off, many thousands were bought by Belgian gun makers who converted them to shotguns, primarily for the African trade, hence the other name, "Zulu shotgun." That is why your gun has Belgian (Liege) proof marks. If they have not been removed, the lock plate should also have the markings of the French arsenal where the gun was originally made.